1. Field of the Invention
The subject invention is directed to pumps for medication systems and, more particularly, reciprocating pumps suitable for use in human implantable medication systems.
2. Description of the Prior Art
Many examples of implantable medical devices intended for unfusing medication intimed dosages are known in the prior art. Early devices were basically pressurized reservoirs from which the medication was to be controllably gated into a catheter. One disadvantage of such devices was that in certain failure modes, they could potentially discharge uncontrolled amounts of medication into the user. To overcome this, such devices typically included somewhat elaborate safety mechanisms. Examples are shown in U.S. Pat. Nos. 4,193,397; 4,221,219; 3,731,681; 4,299,220; 3,894,538; and 3,951,147.
Subsequently, it was recognized that a device wherein the medication in a reservoir could be maintained below atmospheric pressure would be inherently safer since the potential for discharging uncontrolled amounts of medication did not exist. However, the operation of these devices required the medication to be actively pumped from the reservoir. Example are shown in U.S. Pat. No. 4,373,527 and U.S. patent application Ser. No. 439,138, filed Nov. 11, 1982 by Robert E. Fischell. Generally, the medication devices used a battery to charge a capacitor that was discharged to drive the pump solenoid. The pumps were arranged such that closure of the pump solenoid powered the suction stroke for the pump. This stroke would establish a suction in the pump cavity that would draw medication from the reservoir into the pump chamber. When the capacitor current dissipated and the solenoid was de-energized, a spring in the pump would mechanically open the pump solenoid. This stroke would elevate the pressure in the pump chamber and cause the medication therein to be expelled through the output port.
While such prior devices afforded many advantages, a disadvantage was that entrained air in the medication could collect to form a bubble inside the chamber of the pump. Because the solenoid closed relatively quickly in response to the current pulse from the capacitor, the intake stroke was rapid and required the liquid to flow at a high rate through the input valve. This resulted in a large pressure drop between the reservoir and pump chamber and relatively low absolute pressure in the pump chamber. In this way, pumps of prior art devices tended to draw entrained air out of the medication and create bubbles. Due to the relatively high elasticity of air, air bubbles in these devices could potentially compromise their efficiency as well as the accuracy of the medication dosages that they administered.
In the prior art, various proposals have been advanced to avoid or remedy the formation of air bubbles in actively pumped medication systems. For example, in U.S. Pat. No. 4,360,019 a tube between the reservoir and the pump chamber is bent back on itself in a manner intended to block entry of bubbles from the reservoir into the tube. Alternatively, use of a filter in the fluid path between the pump and the reservoir has also been suggested. Similarly, U.S. Pat. No. 4,191,181 described the use of a wicklike member composed of lightly packed glass-like fibers that have sufficient capillary force to prevent gas from entering the fine channels.
However, such highly filtered systems tended to be prone to plugging by contaminant particles and could not be easily cleaned or changed once implanted in a patient. Moreover, they did not address the fact that dissolved or entrained air tended to form a bubble when exposed to low absolute pressures inside the medication pump.
An "inverted pump" operates conversely from the conventional pumps described above. In an "inverted pump" the solenoid is closed to drive the discharge stroke and the spring force drives the intake or suction stroke. During the suction stroke, an inverted pump could maximize absolute pressure in the pump chamber because the spring force could be selected to provide a slow intake of medication. However, none of the inverted pumps of prior art designs would operate in an efficient and repeatable manner suitable for the application of the subject invention.
For example, U.S. Pat. No. 4,152,098 discloses an inverted pump wherein an elastic diaphragm is stretched to permit the movement of a ball during the discharge stroke. The diaphragm then contracts to return the ball to its original position for the suction stroke. A pump of this design would require a diaphragm having sufficient elasticity to accommodate the movement of the ball over its entire stroke. At the same time the pump would require the diaphragm to develop sufficient force to overcome the pressure drop across the inlet check valve, the difference between the pump chamber pressure and the inlet pressure, the weight of moving parts, and friction forces. In a pump constructed in accordance with the design of the U.S. Pat. No. 4,152,098, a diaphragm having sufficient elasticity to accommodate the stroke of the ball, would not develop sufficient force to return the ball in a manner suitable for many applications where repeatability and efficiency are also important.
Accordingly, there was a need in the prior art for an inverted pump suitable for use in medication systems that would operate in an efficient and repeatable manner. Such a pump would maximize absolute pressure in the pump cavity during the intake stroke, thus decreasing the potential for establishing air bubbles in the pump.